1. Field of the Invention
The present invention relates generally to drive shafts for use in intravascular catheter systems having working elements such as ultrasonic transducers or rotating cutting elements. More particularly, the present invention provides an improved drive shaft having a superior combination of column strength, rotational stiffness, flexibility and resistance to permanent deformation or kinking.
2. Description of the Background Art
Arteriosclerosis, also known as atherosclerosis, is a common human ailment arising from the deposition of fatty-like substances, referred to as atheromas or plaque, on the walls of blood vessels. Such deposits occur in both the peripheral blood vessels that feed the limbs of the body and the coronary vessels which feed the heart. When deposits accumulate in localized regions of a blood vessel, stenosis, or narrowing of the vascular channel, occurs. Blood flow is restricted and the person's health is at serious risk.
Numerous approaches for reducing and removing such vascular deposits are known, including balloon angioplasty, in which a balloon-tipped catheter is used to dilate a region of atheroma; atherectomy, in which a blade or cutting bit is used to sever and remove the atheroma; spark gap reduction, in which an electrical spark burns through the plaque; and laser angioplasty, in which laser energy is used to ablate at least a portion of the atheroma. In order to facilitate treatment of the stenosis, it is often desirable to obtain a visual image of the interior of the blood vessel within the region of interest Catheters having imaging elements such as ultrasonic transducers are often used to obtain these images.
In many cases, catheter systems having imaging or interventional devices will include a rotatable drive shaft disposed within the flexible catheter body. In imaging systems, the rotatable drive shaft will typically be connected to an ultrasonic transducer or to a movable mirror associated with an ultrasonic transducer.
An exemplary catheter system having a rotating ultrasonic transducer is disclosed in U.S. Pat. No. 4,794,931, the disclosure of which is incorporated herein by reference. In this system, a drive shaft disposed within a flexible tubular catheter body is used to rotate an ultrasonic transducer about an axis parallel to the catheter body. The ultrasonic transducer is thereby caused to scan a region of the blood vessel in a plane normal to the catheter body.
An alternative imaging catheter system is disclosed in co-pending U.S. patent application Ser. No. 08,006,224, filed on Jan. 19, 1993, the disclosure of which is incorporated herein by reference. In this embodiment, a drive shaft is connected to an ultrasonic transducer by a mechanical linkage which converts rotation of the drive shaft into pivotal oscillation of the transducer about an axis perpendicular to the long axis of the catheter body. The pivotal oscillation of the transducer causes the ultrasonic imaging beam to sweep back and forth through a plane to image a region forward of the distal end of the catheter body.
Another catheter system, which may include either an imaging or interventional work element connected to a rotating drive shaft, is disclosed in U.S. patent application Ser. No. 07/976,228, filed on Nov. 13, 1992, the disclosure of which is incorporated herein by reference. This "common lumen" catheter system includes a proximal region having multiple lumens for carrying guidewires or various work elements and a reduced profile single lumen distal region adapted to enter narrow and tortuous. regions.
Drive shafts suitable for use in applications such as those described above must satisfy several demanding criteria, some of which conflict. First, a catheter drive shaft must have sufficient column strength to allow for advancement of the work element within the catheter by pushing the drive shaft from the proximal end. A drive shaft with inadequate column strength will tend to collapse or buckle. To provide adequate column strength, the drive shaft material should have a high elastic modulus.
Second, a rotating drive shaft must have adequate torsional rigidity. In the case of an ultrasonic transducer as an imaging element, the need for torsional rigidity is critical. An imaging catheter system typically includes equipment for displaying an image of the interior of the blood vessel. This display equipment is usually synchronized with the means for rotating the drive shaft, typically a drive motor connected to the proximal end of the drive shaft. For high quality imaging, the drive shaft must faithfully convert constant speed rotation of the motor into constant speed rotation of the imaging element. If the drive shaft twists or winds up on itself, the ultrasonic transducer will lose synchronization with the display equipment, and the displayed image may be blurred or degraded to the point of uselessness. For adequate torsional rigidity, the drive shaft material should have a high shear modulus.
Third, to allow for advancement of the work element into narrow and twisting regions of the patient's vascular system, at least a distal portion of the drive shaft should be reasonably flexible. This requirement is obviously at odds with the need for the drive shaft to have a high column strength. To address these conflicting requirements, some drive shafts are made in sections. Such a multiple section drive shaft would have a relatively stiff segment at its proximal end to allow for adequate pushability, and a relatively flexible segment at its distal end to allow bending for entry into narrow and tortuous regions.
Fourth, a rotating catheter drive shaft should be highly resistant to permanent deformation, or kinking, along its length. If a permanent bend develops in the drive shaft, the ultrasonic transducer will be subjected to a whipping action at the distal end of the catheter and uniform rotation of the imaging element will become impossible. This will drastically degrade the displayed image.
Finally, to the extent permitted by the other requirements, the drive shaft should have a small diameter to permit entry of the work element into and through narrow blood vessels, vessels which typically will have been even further narrowed by deposits of plaque on the vessel walls.
Past drive shafts have often been made of wound or braided wire cables. Some of these drive shafts have combined segments of varying flexibility, sometimes including a rigid metal member for stiffness at the proximal end. One such multiple segment drive shaft is disclosed in U.S. Pat. No. 5,108,411, the disclosure of which is incorporated herein by reference.